S. STOICA
8
/ mN (VlN – mixing parameters between light (active) and
heavy (sterile) neutrinos); and 3) exchange of neutrinos
with mass of the of the order of the energy decay process
(mN
2 ~ q2). In this case they can be produced on their
mass shell and the decay rates are enhanced due to the
resonant effect associated to their decay widths ΓN → R
~ ΣNVlNVl’N/ ΓN. For the first two cases the decay rates
are quite small: in the first case due to the smallness of
both the mixing parameters between the active neutrinos
and of the neutrino mass (of ~ 1 eV), and in the second
case due to the smallness of the mixing parameters be-
tween the ligh (active) and heavy (sterile) neutrinos di-
vided by the large mass of the heavy neutrino. The
branching ratios (Br) in these cases are of the order of
10-20 -10-31 and, at the present LHC luminosities, they can
not be detected. The third case is quite interesting since
the theoretical predictions for the Br for these resonant
decay processes become at reach of the present and next
future high energy experiments.
Until now several LNV processes at high energy have
already been investigated. Their non-observation has set
bounds on the corresponding Br and, further, on the neu-
trino mixing parameters. In the following we shortly
present the most important such investigations performed
at LHC experiments at CERN.
ATLAS experiment [14] has performed an inclusive
search of events with two isolated leptons (e or μ) having
the same electric charge. The data are selected from
events collected from pp collisions at sqrt(s) = 7 TeV by
the ATLAS detector and correspond to an integrated lu-
minosity of 34 pb-1. The spectra in dilepton invariant
mass are compared to SM predictions. No evidence is
found for contributions beyond those of the SM. Limits
are set on the cross-section in a fiducial region for new
sources of same-sign high-mass dilepton events in the ee,
e and channels. Four models predicting same-sign
dilepton signals are constrained: two descriptions of Ma-
jorana neutrinos, a cascade topology similar to super-
symmetry or universal extra dimensions, and fourth gen-
eration down-type quarks. Assuming a new physics scale
of 1 TeV, Majorana neutrinos produced by an effective
operator V with masses below 460 GeV are excluded at
95% CL. A lower limit of 290 GeV is set at 95% CL on
the mass of fourth generation down type quarks.
CMS experiment [15] has searched for events with
same-sign isolated dileptons (ee, e, ,
, ). The
searches used an integrated luminosity of 35 pb-1 of pp
collision data at a ECM of 7 TeV collected by the CMS
experiment at the LHC. The observed numbers of events
agree with the SM predictions, and no evidence for new
physics was found. To facilitate the interpretation of the
data in a broader range of new physics scenarios, infor-
mation on the event selection, detector response, and
efficiencies is provided.
LHCb experiment has investigated several LNV proc-
esses of meson and tau decays. A first search of same
sign dileptons was performed in the decays of B+ → K-
(π-) μ+μ+ at an integrated luminosity of 36pb-1 [16]. No
signal was observed in either channels. They set limits of
the Br for the two channels as follows: Br(B+ → K- μ+ μ+ )
< 5.4·10-8 and Br( B+ → π- μ+ μ+ ) < 5.8 · 10-8 at 95% CL,
which improves the previous existed limits by factors of
40, 30, respectively. Another analysis was performed for
the B- decays into same sign di-muon channels at an in-
tegrated luminosity of 380 pb-1 [17]. Also, no signal was
observed beyond the SM and limits were set for the
channels Br(B- → D+ μ- μ-) < 5.6·10-7 and B(B- → D*+ μ-
μ- ) < 4.1 x 10-6 at 90% CL. Besides these B meson
channels a first search for tau decays was also performed
[18]. Particularly, such an investigated process was - →
μ+ μ- μ-, which is in the same time a Lepton Flavor Vio-
lating process. The analysis was done using 1.0 fb-1 of
data collected in 2011 at (s)1/2 = 7 TeV. The upper limit
for the Br was Br < 7.8 ·10-8 at 95% CL. These studies
performed at LHC experiments will be certainly im-
proved in the next future with a new set of data at an in-
creased luminosity. Besides these already investigated
channels, there are many others that merit to be investi-
gated, according to theoretical estimations for the Br. For
example, the hyperon decay channels are at present
weakly constraint (Br ~ 10-3 – 10-4). In addition, there are
very recent theoretical estimations which show that
4-body decay channels, like decays of neutral mesons (B,
D), or the tau decay channel - → μ- μ- π+ can provide
us with even more stringent bounds on Br and neutrino
mixing parameters between muon flavor and sterile fla-
vor (N) [19]. Thus, the study of the LNV channels at
high energies opens an interesting direction of investiga-
tion at LHC experiments and superB factories in the next
future.
5. Conclusions
Recent neutrino oscillation experiments have convinc-
ingly shown that neutrinos are massive particles and they
mix. This is the first evidence that extends our under-
standing on the SM and encourages us to search for BSM
physics. The large majority of BSM theories involve
massive Majorana neutrinos which imply non-conserva-
tion of the LN. Concerning the neutrino properties we
still do not know important issues as the scale of their
absolute mass and the mass hierarchy, the mechanism of
their mass generation, the nature of neutrinos (are they
Dirac or Majorana particles?), the number of neutrino
flavors, etc. The LNV processes can shed light on these
issues, that is why there is a great interest to search for
such processes. At low energy there is the 0ββ decay, a
process which is intensively studied both theoretically
and experimentally. Theoretically, a key challenge is to
Copyright © 2013 SciRes. OJM